PSI - Issue 10

G.V. Seretis et al. / Procedia Structural Integrity 10 (2018) 249–256 G.V. Seretis et al. / Structural Integrity Procedia 00 (2018) 000 – 000

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GNP and the regions before and after it. Fig.6 shows a region of the matrix with GNPs around a pore. The linear height distribution of this figure corresponds to the line marked with a red arrow, which includes same regions as before, i.e. a GNP and the regions before and after it. The height distribution before and after the peak that corresponds to a GNP is described by a constant height profile, indicating that no voids were found at the GNPs boundaries. Therefore, it can be concluded that the embodiment of the GNPs in the epoxy matrix was proper. To cross-check the existence of GNPs in the investigated regions EDS analysis was carried out in each one of them.

Fig. 5. AFM micrograph showing four GNPs in the epoxy matrix together with the linear height profile for the area around the upper-left GNP (red arrow).

Fig. 6. AFM micrographs showing four GNPs around a pore of the epoxy matrix together with the linear height profile for the area around a GNP (red arrow). 3.2. Effect of GNPs content on T2x2 specimens For the specimens reinforced with Twill 2×2 fabric, a GNPs content equal to 1% w.t. leads to considerable increase in Ultimate Tensile Strength (UTS), as can be seen in Fig.7. The fibrils within a fiber perform as a filter for the GNPs. Therefore, the GNPs stack on the fibrils’ surface, leading to increase their performance when the GNPs content is low (Seretis et al. (2017a)). For further increase of the GNPs content, UTS progressively decreases. This indicates a pro gressive increase of the inter-yarn friction while GNPs content increases. Due to this increasing frictional behavior, the wear of the fibrils is getting progressively greater, leading to the failure of the specimens at lower tensile loads. However, this constantly increasing frictional behavior seems to have a positive effect on the tensile strain. Indeed, the strain at break progressively increases until the GNPs content reaches 4% w.t. and, subsequently, it shows a reduc tion trend (Fig.8). Since the fibers can rotate in the in-plane level until they reach a specific angle (Seretis et al. (2015; 2016)), constant for a specific kind of specimens, by increasing the GNPs content, and consequently the inter-yarn fric tion, the fibrils reach this specific angle value at greater tensile stain. On the other hand, the greatly increased friction values in the case of 5% w.t. GNPs seem to cause early collapse of the fibrils, before they reach the locking angle.

3.3. Effect of GNPs content on UD specimens

The structure of the Uni-Directional fabric was constituted by a high-density warp direction (main direction) and a low-density weft direction. Therefore, in this fabric the weft direction was used only to keep the structure of the warp direction.